Least invasive surgical techniques have gained significant popularity because of their ability to accomplish outcomes with reduced patient pain and accelerated return of the patient to normal activities. Arthroscopic surgery, in which the intra-articular space is filled with fluid, allows orthopedic surgeons to efficiently perform procedures using special purpose instruments designed specifically for arthroscopists. Among these special purpose tools are various manual graspers and biters, powered shaver blades and burs, and electrosurgical devices. During the last several years, specialized arthroscopic electrosurgical electrodes called ablation electrodes or ablators have been developed. Exemplary of these instruments are ArthroWands manufactured by Arthrocare (Sunnyvale, Calif.), VAPR electrodes manufactured by DePuy Mitek, a subsidiary of Johnson & Johnson (Westwood, Mass.) and electrodes by Smith and Nephew, Inc. (Andover, Mass.). These ablation electrodes differ from conventional arthroscopic electrosurgical electrodes in that they are designed for the bulk removal of tissue by vaporization, rather than the cutting of tissue or coagulation of bleeding vessels. While standard electrodes are capable of ablation, their geometries are not efficient for accomplishing this task. The tissue removal rates of ablation electrodes are lower than those of arthroscopic shaver blades, however, electrosurgical ablation electrodes or “ablators” are used because they achieve hemostasis (stop bleeding) during use and are able to efficiently remove tissue from bony surfaces. Ablation electrodes are used in an environment filled with electrically conductive fluid.
During ablation, current flow from the ablator into the conductive fluid heats the fluid to its boiling point. Heating of the conductive fluid is proportional to the density of electrical current flowing from the electrode into the fluid. Regions of high current density will experience higher rates of heating as compared to regions of low current density. Such regions of high current density typically arise at the corners and edges of the electrode. Steam bubbles form first at the edges of an ablator but eventually cover virtually the entire surface of the electrode. When a steam bubble reaches a critical size, arcing occurs within the bubble. If the bubble intersects with tissue, arcing occurs between the electrode and the tissue thereby vaporizing a portion of the tissue. A train of sparks often occurs within the bubble with the train ending when the bubble grows too large or the tissue enclosed in the bubble is evaporated, at which point conditions within the bubble become unfavorable for sparking.
During ablation, water within the target tissue is vaporized. Because volumes of tissue are vaporized rather than discretely cut out and removed from the surgical site, the power requirements for ablation electrodes are generally higher than those of other arthroscopic electrosurgical electrodes. The efficiency of the electrode design and the characteristics of the Radio Frequency (RF) power supplied to the electrode also affect the amount of power required for ablation. Electrodes with inefficient designs and/or powered by RF energy with poorly suited characteristics will require higher power levels than those with efficient designs and appropriate generators. Because of these factors, the ablation power levels of devices produced by different manufacturers vary widely with some requiring power levels significantly higher than those commonly used by arthroscopists. For example, ablation electrode systems from some manufacturers may use up to 280 Watts, significantly higher than the 30 to 70 Watt range generally required by other arthroscopic electrosurgical electrodes.
During artroscopic electrosurgery, all of the RF energy supplied to the electrode becomes heat, thereby raising the temperature of the fluid within the joint and the temperature of adjacent tissue. And, until the introduction of ablation electrodes, the temperature of the fluid within the joint was not of concern to the surgeon. However, fluid temperature is a primary concern during the use of ablation electrodes due to the higher power levels at which they generally operate and the longer periods of time that they are energized. Standard arthroscopic electrosurgical electrodes are usually energized for only brief periods, generally measured in seconds, while specific tissue is resected or modified, or a bleeder coagulated. In contrast, ablation electrodes are energized for longer periods of time, often measured in minutes, while volumes of tissue are vaporized.
The temperature of the fluid within the joint is critical since cell death occurs at 45° C., a temperature easily reached with high-powered ablators if fluid flow through the surgical site is insufficient. Patient injury can result and such injuries have been documented.
The likelihood of thermal injury is strongly affected by the amount of power supplied to the ablator. This, in turn, is determined by the efficiency of the ablator and the speed with which the surgeon desires to remove tissue. A highly efficient ablator will allow the surgeon to remove tissue at desirably high rates, while requiring low levels of power input. Under these conditions the likelihood of thermal injuries is reduced significantly.
Ablation electrodes are produced in a variety of sizes and configurations to suit a variety of procedures. For example, ablators for use in ankle, wrist or elbow arthroscopy, are smaller than those used in the knee or shoulder. In each of these sizes, a variety of configurations are produced to facilitate access to various structures within the joint being treated. These configurations differ in the working length of the electrode (i.e., the maximum distance that an electrode can be inserted into a joint), in the size and shape of their ablating surfaces and in the angle between the ablating face and the axis of the electrode shaft. Electrodes are typically designated by the angle between a normal to the ablating surface and the axis of the electrode shaft, and by the size of their ablating surface and any associated insulator.
Primary considerations of surgeons when choosing a particular configuration of ablator for a specific procedure include its convenience of use (i.e., the ease with which the instrument is able to access certain structures) and the speed with which the ablator will be able to complete the required tasks. When choosing between two configurations capable of accomplishing a task, surgeons will generally choose the ablator with the larger ablating surface so as to remove tissue more quickly. This is particularly true for procedures during which large volumes of tissue must be removed. One such procedure is acromioplasty, the reshaping of the acromion. The underside of the acromion is covered with highly vascular tissue which may bleed profusely when removed by a conventional powered cutting instrument such as an arthroscopic shaver blade. Ablation electrodes are used extensively during this procedure since they are able to remove tissue without the associated bleeding which can obscures the surgeon's view of the site. Ablation in the area under the acromion is most efficiently accomplished using an electrode on which a line normal to the ablating surface is approximately perpendicular to the axis of the ablator shaft. Such an electrode is designated as a “90 Degree Ablator” or a “side effect” ablator. Exemplary of such electrodes are the “3.2 mm 90 Degree Three-Rib UltrAblator” by Linvatec Corporation (Largo, Fla.), the “90 Degree Ablator” and “90 Degree High Profile Ablator” by Smith and Nephew (Andover, Mass.), the “Side Effect VAPR Electrode” by DePuy Mitek, a subsidiary of Johnson and Johnson, and the “3.5 mm 90 Degree Arthrowand,” “3.6 mm 90 Degree Lo Pro Arthrowand,” and “4.5 mm 90 Deg. Eliminator Arthrowand” by Arthrocare Corporation.
A recent improvement to ablation electrodes is the addition of means of aspiration to remove bubbles and debris from the surgical site. During electrosurgery in a conductive fluid environment, tissue is vaporized, thereby producing steam bubbles which may obscure the view of the surgeon or displace saline from the area of the intra-articular space which the surgeon wishes to affect. In the case of ablation (bulk vaporization of tissue), the number and volume of bubbles produced is even greater than when using other electrodes since fluid is continually boiling at the active electrode during use. Ideally, flow through the joint carries these bubbles away; however, in certain procedures this flow is frequently insufficient to remove all of the bubbles. The aspiration means on an aspirating ablator removes some bubbles as they are formed by the ablation process, and others after they have collected in pockets within the joint. The ablator aspiration means is connected to an external vacuum source which provides suction for bubble evacuation.
The aspiration means on currently available ablator products may be divided into two categories according to their level of flow. High-flow ablators have an aspiration tube, the axis of which is coaxial with the axis of the ablator rod or tube, which draws in bubbles and fluid through its distal opening and/or openings cut into the tube wall near its distal tip. High-flow ablators may decrease the average joint fluid temperature by removing heated saline (waste heat since it is an undesirable biproduct of the process) from the general area in which ablation is occurring. The effectiveness of the aspiration, both for removal of bubbles and for removal of waste heat, will be affected by the distance between the opening through which aspiration is accomplished and the active electrode. The distal tip of the aspiration tube is generally several millimeters distant proximally from the active electrode so as to not to obstruct the surgeon's view of the electrode during use. Decreasing this distance is desirable since doing so will increase the effectiveness of the aspiration. However, this must be accomplished without limiting the surgeon's view or decreasing the ablator's ability to access certain structures during use. Examples of high-flow aspirating ablators systems include the Three Rib-Aspirating ablators by Linvatec Corporation and the 2.3 mm and 3.5 mm Suction Sheaths for the VAPR system by DePuy Mitek, the sheaths being used with standard VAPR ablation probes.
Arthrex, Inc. (Naples, Fla.) markets aspirating ablators having an aspiration means wherein the aspiration port is in the distal-most surface of the device, and the aspiration path is through the device. These devices have higher flow rates than low-flow ablators, though less than the high-flow models previously herein described.
Low-flow ablators are those which aspirate bubbles and fluid through gaps in the ablating surfaces of the active electrode and convey them from the surgical site via means in the elongated distal portion of the device. Current low-flow ablators require increased power to operate as effectively as a nonaspirating or high-flow aspirating ablators because the low-flow aspiration is drawing hot saline from the active site of a thermal process. In the case of low-flow ablators, the heat removed is necessary process heat rather than the waste heat removed by high-flow ablators. Because of this, aspirating ablators of the low-flow type generally require higher power levels to operate than other ablators thereby generating more waste heat and increasing undesirable heating of the fluid within the joint. Typical of low-flow aspirating ablators are those produced by Arthrocare and Smith and Nephew.
Each of these types of aspirating ablation electrodes has its drawbacks. In the case of high-flow aspirating ablators, the aspiration tube increases the diameter of the device thereby necessitating the use of larger cannulae, which, in turn, results in an increase in wound size and often an increase in patient pain and recovery time. In the case of low-flow aspirating ablators, the devices decrease the efficiency of the probes since process heat is removed from a thermal process. This decreased efficiency results in decreased rates of tissue removal for a given power level. This results in increased procedure times or necessitates the use of higher power levels to achieve satisfactory tissue removal rates. High power levels are undesirable as they cause increased heating of the fluid at the site and thereby increase the likelihood of thermal injury to the patient.
It is an object of this invention to produce an electrosurgical ablation electrode which aspirates through the ablating portion of the active electrode and has increased ablation efficiency as compared to existing ablation electrodes which aspirate through the active electrode.